An engine coupled to an automatic transmission may be stopped and restarted to conserve fuel during conditions of low engine torque demand. If the engine is started while the transmission is in neutral, the speed of the engine at restart may not be of much concern so long as engine starting speed is somewhat repeatable. However, if the engine is started while the transmission is in gear, the torque transmitted by the engine to the transmission during engine run-up from crank to idle speed may be objectionable to the driver if more than a desired amount of engine torque is transferred from the engine to the transmission during engine restart. Additionally, if the driver requests torque from the engine to accelerate a vehicle shortly after an engine is started in gear, delivery of engine torque to the vehicle wheels may not be smooth and may be objectionable to the driver.
The inventors herein have recognized the above-mentioned disadvantages of restarting an engine coupled to a transmission that is in gear and have developed a method for starting an engine coupled to a transmission, comprising: cranking the engine from stop; releasing an off-going clutch of the transmission to downshift the transmission during engine speed run-up from cranking to idle speed; and applying an on-coming clutch of the transmission in response to a reduction of a transmission input torque brought about via the off-going clutch.
Torque that is transmitted from an engine and output from a transmission during engine starting may be controlled via starting the engine in a first gear that is applied by a first transmission clutch (e.g., the off-going clutch). In particular, starting the engine while the transmission is in a higher gear having a lower gear ratio (e.g. fourth gear having a ratio of 1:1) may limit the amount of engine torque that is transmitted through the transmission to wheels of a vehicle. Further, the transmission may be shifted to a second lower gear having a higher gear ratio (e.g., first gear having a ratio of 3:1) that is applied by a second transmission clutch (e.g., the on-coming clutch) when a driver requests that the engine accelerate or launch a vehicle from stop. Shifting the transmission to a lower gear having a higher gear ratio may improve vehicle launch. In addition, the torque transfer from the engine to the vehicle wheels may be improved by applying the on-coming clutch as torque across the off-going clutch is decreased. In this way, torque can be transferred from one gear set to the next gear set such that torque may flow through the transmission without a substantial torque decrease at the vehicle wheels during the downshift, thereby improving vehicle launch.
The present description may provide several advantages. Specifically, the approach may improve vehicle launch for engines that are automatically stopped and restarted. Further, the approach may improve vehicle emissions since the engine may be started in a more repeatable manner. Further still, the approach may improve vehicle drivability by improving the way torque is transmitted from the engine to vehicle wheels.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.